The "Union of the Principalities" refers to the political unification of the Principalities of Moldavia and Wallachia in 1859, which laid the foundation for modern Romania. This union was significant as it marked the beginning of a centralized Romanian state, promoting economic and cultural development. The union was achieved through the election of Alexandru Ioan Cuza as the ruler of both principalities. His leadership helped to implement reforms that modernized the administration, military, and education systems, ultimately leading to the official establishment of Romania in 1866.